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One fact about black holes has long puzzled me: I’ve read in a number of articles that black holes will from time to time violently discharge a mix of matter and radiation. How is this possible? Once a black hole sucks anything past its event horizon, that material never gets back out. Hence the term black hole – even light can’t get back out.
Yesterday, I found the answer while reading Steve Nadis’ article in September’s Discover magazine: “To the Edge and Back.” The article mainly describes the Event Horizon Telescope currently under development, a fascinating collaboration between astronomers and observatories around the world. The answer to my long-held question about black hole “emissions” comes in the article’s introduction.
It turns out that the “discharged” material never made it past the event horizon. When a black hole encounters more matter than it can consume, all the matter sucked towards the hole by its huge gravitational pull causes an enormous “traffic jam” that keeps most of the matter from actually making it down into the hole. Excess matter keeps piling up, and the pressure grows.
Atoms and small particles grind against each other, heating to billions of degrees. Boyle’s law lays out the relationship between temperature, pressure, and volume. A rapid increase in the first two will force a change in the third. Volume must expand, but it can’t expand into the black hole due to the clog, so it shoots out into space at close to the speed of light.
At least, that’s the theory. If the Event Horizon Telescope project can come together by the time the black hole in the center of our galaxy encounters gas cloud G2 in the next year or so*, we might be able to watch the process happen and learn more about it.

*Yes, I know the actual event happened light years ago, but we don’t get to watch it until a year or so from now when the light from the event reaches Earth.